Rainwater Collection Calculator
Estimate roof runoff from a rainfall event using your own collection assumptions.
Your result
Estimated collected water
1486 L
Fits in tank
1000 L
Potential overflow
486 L
Rainfall volume and storage
One millimetre of rainfall across one square metre equals one litre. Multiply horizontal roof area by rainfall depth and collection efficiency. Efficiency accounts approximately for losses; the default 80% is an illustrative assumption, not a measured value.
This is an event-volume estimate, not a prediction of local rainfall or a plumbing design. It does not include stored water already in a tank, consumption, first-flush details or local restrictions. Collected roof water is not assumed safe to drink.
Frequently asked questions
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